Thermal ink jet printing has been described in many technical publications, and one such publication relevant to this invention is the Hewlett Packard Journal, Volume 36, Number 5, May 1985, incorporated herein by reference.
In the art of thermal ink jet printing, it is known to provide a plurality of electrically resistive elements on a common thin film substrate for the purpose of heating a corresponding plurality of adjacent ink reservoirs during the ink ejection and printing process. Using such an arrangement, the adjacent ink reservoirs are typically provided as cavities in a barrier layer above the substrate for properly concentrating thermal energy emanating from the resistive elements to predefined volumes of ink. Also, a plurality of ink ejection orifices are provided above these cavities and provide exit paths for ink during the printing process.
In constructing the above type of printhead assembly, one practice has been to drill vertical holes in a common substrate in order to provide ink flow paths from a common ink reservoir to the individual reservoir cavities within the barrier layer. However, the use of multiple holes (vertical cylindrical channels) in a single substrate possesses several disadvantages. One of these disadvantages is that the boring bit used for drilling holes in the substrate places a substantial pressure on the substrate material and thus can cause fracturing of this material. On the other hand, if laser drilling is utilized, the laser beam will leave channels with fratured side walls as a result of heating, and thus produce a weakened substrate structure.
The per se creation of mulitiple vertical channels in the silicon substrate weakens the printhead structure, and with some types of prior art printhead structures, these channels are used to provide ink flow to a plurality of resistive heater elements located at different distances from the channels. In such a structure, these varying ink-flow distances produce corresponding different pressure drops in the ink flow paths. That is, the pressure drop along a liquid ink flow path is proportional to the cube of the distance of the path. This fact has sometimes resulted in pressure drops over large ink flow distances sufficiently great as to prevent adequate vaporization during ink jet propulsion from the ink jet orifice.
Another disadvantages of using small diameter vertical channels to supply ink to the ink reservoirs is tha these channels simply do not have the capacity to adequately respond to certain ink volume demands at the required increasingly higher frequencies of operation.
A further disadvantage of using a plurality of ink flow channels in a common substrate is that they normally require a special routing of conductive leads on the substrate surface. In addition to the added costs associated with this special routing, this requirement also greatly reduces the achievable packing density because of the surface area required to accomodate such special routing schemes.